An electrolytic cell and process for winning aluminum from alumina in which a crust of cryolite and alumina is used as an electrode.
For over a century, the Hall-Heroult electrolytic cell has been used for making aluminum from alumina. Virtually universally, the electrolyte includes cryolite (Na3AlF6) along with the alumina, and the anodes are made of a high percentage of carbon.
A succinct description of the basic process is found in column 1, lines 35-46 of Dell""s U.S. Pat. No. 3,303,119: xe2x80x9cIn operation, a mixture of alumina and cryolite (usually with one or more other fluorides) is provided in the cell cavity, and an electric current is passed through the cell. The resistance of the alumina-cryolite charge to the passage of current produces sufficient heat to fuse the same, and form a molten electrolyte or bath, which may then be considered as a solution of alumina in molten cryolite, Aluminum is electrolyzed from the solution, depositing as a molten layer on the cathode, while oxygen passes to the anode. A crust of frozen electrolyte forms on the surface of the bath (which is usually at a temperature of about 970xc2x0 C.) and this crust is usually covered over with some undissolved alumina.xe2x80x9d Also, xe2x80x9cOperating data confirm that approximately 0.4 pound of carbon per pound of aluminum metal produced is necessarily consumed in this manner . . . As the anode carbon is consumed, the anode is lowered into the bath by mechanical or automatic meansxe2x80x9d (column 1, lines 53-61).
Anodes made of various carbon compositions are widely used in the cells, but have several significant disadvantages.
Carbon monoxide and carbon dioxide formed around the carbon anodes tend to block and reduce the passage of current to the anode. In turn, this increases the voltage of the current that passes through the anode, which may cause the formation of toxic gases such as fluorine, carbon tetrafluoride, C2F4, and hydrogen fluoride. This highly undesirable phenomenon is commonly known as the xe2x80x9canode effect.xe2x80x9d
Carbon anodes are often made from carbon sources and binders from the byproducts of coke production or other coal processes, such as tar and pitch, which are environmentally difficult materials to work with. Large numbers of anodes are used at any given time in the aluminum industry. Since they are consumed in the aluminum production process, they must be replaced frequently, which means the sheer quantity of raw material and finished anode product is an environmental problem.
In addition, the manufacture of carbon anodes is labor intensive and expensive. Because of the cost of the anodes, the spent xe2x80x9cbuttsxe2x80x9d are reused, but the reuse of the butts requires washing and other treatment which results in a waste water stream containing such toxic materials as cyanide, benzene, toluene, and other organics found in tars and pitch.
Rapp, in U.S. Pat. No. 6,039,862, points out as an additional disadvantage of carbon electrodes the generation of xe2x80x9cgreenhouse gas,xe2x80x9d and alludes to a search for a non-consumable electrode. He estimates the cost of consumable carbon anodes amounts to 14.4% of the cost of producing aluminum.
In U.S. Pat. No. 3,787,310, Johnson reviews the previous patent literature on coating and impregnating carbon anodes in order to reduce erosion of the anodes. He relates that it has been common in the past to splatter molten bath on parts of the anode, and to dust cryolite powder on the red hot anodes, to which it will adhere. He proposes coating an otherwise more or less conventional carbon anode with cryolite. See also Johnson""s U.S. Pat. No. 3,787,300 and Skantze et al U.S. Pat. No. 3,236,753, describing an xe2x80x9cimpermeablexe2x80x9d coating of cryolite including an excess of aluminum fluoride and a minor amount of alumina, used to protect a carbon anode against erosion in the cell. In Example 1 of the Skantze et al patent (issued in 1966), it is said that, although the coating appeared to be generally beneficial, xe2x80x9cthe side coating peeled off because the coating fused to the crust of electrolyte on top of the molten bath of electrolyte in the cell.xe2x80x9d
The aluminum industry consumes large quantities of power, which subjects it to criticism for pollution incident to electrical power generation as well as its own contributions inevitable in the use of carbon anodes. This invention provides a way to eliminate the carbon anodes.
I have invented a process for the electrolytic manufacture of aluminum which does not require the use of a carbon anode. Briefly, my process uses solid cryolite/alumina as the anode in contact with the molten cryolite/alumina electrolyte.
The normal operation of my process is similar to conventional processes in one aspect, that a cryolite/alumina mixture is subject to an electric current and aluminum is separated at the cathode. The cathode may be any conventional material or metal and is situated in the bottom of the cell as is conventional. In another, major, aspect, however, it is quite different in that there is no carbon anode. The anode is solid cryolite or a solid cryolite/alumina mixture. This solid anode comprising cryolite is connected to the power system by a metallic connectorxe2x80x94that is, a conductive metal such as steel, cast iron, or titanium.
To begin the process, a piece of solid cryolite or cryolite/alumina is connected to the power by a metallic connector.